An HVDC system is a system that converts alternating current (AC) power, generated by a power plant, into direct current (DC) power, transmits the DC power, converts the DC power back into AC power at a receiving end, and supplies the AC power to a load. Such a HVDC system may allow power to be efficiently and economically transmitted by a voltage increase, which is an advantage of AC power transmissions, and may overcome various shortcomings of AC power transmissions (such as efficiency of long distance transmission, and the like).
For controlling an effective power transmission amount of the HVDC system, a firing angle of the converter (rectifier or inverter) included in the HVDC system may be controlled. However, such control may cause reactive power variations and vibrations of the converter, and such reactive power variations and vibrations may cause problems such as a rectification failure of a converter.